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The invention pertains to printing and more particularly to digital imaging of media sleeves, a field also known as Computer-to-Plate imaging.
In many types of printing, particularly flexographic printing, offset printing, gravure printing and screen-printing, there is an advantage in using media sleeves as printing elements instead of plates wrapped around printing cylinders. Media sleeves, in the form of seamless sleeves, allow printing of continuous patterns. The use of media sleeves in general allows printing presses to operate with faster printing job turnover.
However, before a media sleeve can be mounted on a printing press, it first has to be imaged and processed, although some materials are available today that do not require processing. Prior art imaging devices for imaging such media sleeves were built in the general form of a lathe. Such machines have a mandrel on which a media sleeve can be mounted, a fixed headstock for driving the media sleeve, a moveable tailstock for supporting the media sleeve, and a traveling imaging head. In these systems the traveling tailstock typically moves on tracks in order to accommodate sleeves of different lengths.
Presses are often equipped with a cylinder of a fixed size, or have a cylinder-mandrel arrangement that allows media sleeves of differing diameter to be accommodated by mounting a different cylinder on the press. A single imaging machine typically serves a number of different presses. As a result media sleeve sizes have to be changed more often on an imaging machine than on a press. The changing of the cylinder-mandrels is therefore a major problem on the imaging machines, where the changes are frequent, and the cumbersome changing process, characterized by the extensive handling of bulky and heavy cylinder-mandrels, becomes a major operational bottleneck.
On a typical imaging machine, replacing a media sleeve involves removing the cylinder-mandrel from the exposure machine and removing the media sleeve from the cylinder-mandrel. Typically the media sleeve is removed from the mandrel by connecting a pneumatic supply to the cylinder-mandrel and pressurizing the inside, causing air to leak out from small holes under the media sleeve. Such airflow expands the media sleeve and creates an air bearing, allowing the media sleeve to slide off the cylinder-mandrel and be replaced by another sleeve to be imaged.
In the field of computer-to-plate (CTP) imaging, lasers are employed to image the media sleeves. The generally high imaging performance of these systems is achieved through comparatively intricate optical arrangements involving laser arrays and/or a variety of light modulation devices such as light valves. These arrangements often result in stringent requirements on the tracking, alignment and, in particular, focus between the media sleeve and the imaging head.
The excessive mounting, demounting and handling of heavy mandrels is therefore not only an operational bottleneck, but also presents a mechanical danger to, what is in reality, a precision optics machine. It is furthermore important that the media sleeves, when mounted on the mandrels, should rotate as perfectly as possible without any run-out or vibrations, in order to facilitate optimal imaging by the precision optics head.
A typical imaging machine therefore is subject to a higher frequency of media sleeve changes than a typical press served by the same imager, whilst also having to maintain very high optical precision. Clearly the media sleeve mounting requirements of an imaging machine present rather different challenges from those of mounting the imaged media sleeve on a press, and there is therefore a requirement for an arrangement and method that can limit the handling of bulky mandrels and yet maintain the accuracies demanded by the precision optics of the imaging machine.
Prior art computer-to-plate imagers are based on the changing of entire bulky mandrels in order to facilitate a change in media sleeve diameter. While various mandrel arrangements have been proposed for machine tools and presses, these have not addressed the rapid interchangeability and precision needs of the latest generation of computer-to-plate imagers, typically equipped with laser-based imaging heads.
The invention provides a computer-to-plate method and apparatus for imaging media sleeves by employing a laser-based imaging head in conjunction with a two-stage mandrel comprising an expandable arbor and an intermediate sleeve. Most of the bulk of the mandrel is contained in the expandable arbor, which may be expanded hydraulically in order to mechanically engage the intermediate sleeve. The media sleeve to be imaged is mounted on the intermediate sleeve by conventional means. The method and apparatus of the invention makes possible the rapid changing between different sizes of media sleeves without requiring the handling of bulky mandrels and to minimize potential damage to the precision optics imaging head of the apparatus, whilst maintaining the mounting precision.